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  #11  
Old 02-21-2016, 04:21 PM
Jbon Jbon is offline
 
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Quote:
Originally Posted by aerhed View Post
. . . A carb with a rattly shaft will suck air from the sides, that's air that doesn't go past the idle ports.
Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.

What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.

I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.

Do you think this is the problem?

Anyone know a good carburetor shop in the DFW area?



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  #12  
Old 02-21-2016, 09:07 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Jbon View Post
Okay, I finally got out to the airport today to do some more detective work. I hooked up my (clean) shopvac and pressurized the air inlet. The intake tubes were tight, but I'll be changing the gaskets anyway because of age.

What was more troubling was a pretty good leak around the throttle butterfly shaft where it protrudes from the opposite side of the carb. The shaft is not wobbly, but I can push it in and out slightly when the butterfly is wide open. Doesn't move when shut. The air leak varied a bit, but was quite noticeable. I also saw much smaller bubbles around the mixture screw.

I described this to one of our local A&P's, and he didn't seem to think it was a big deal, but your comment on how a leak could affect the idle circuit makes sense to me. Again, the engine idles great at around 1000 RPM, maybe a bit rougher at 700, but it bothers me that I'm not seeing the RPM rise I should at shutdown, and that when I made the large adjustment, to get a 40 RPM rise, it didn't stick.

Do you think this is the problem?

Anyone know a good carburetor shop in the DFW area?



The shaft bushing interface is not designed to be air tight and contains no seal. Air leak there is expect and is not causing a problem. the shaft has no mechanism to stop side to side movement other than the butterfly valve in the throat. You should have side to side play.

Do some more research on this before you get too concerned. Many mechanics tune idle by sound and performance. If your engine is running well don't concern yourself too much. Go research mixture adjustment procedures. I have seen some good articles with more detailed advice. It may just be the testing procedure that you are using.

Larry
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  #13  
Old 02-21-2016, 09:16 PM
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bruceh bruceh is offline
 
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Mike Busch has a good article on in-flight diagnostics for finding/diagnosing induction leaks here. It assumes you have full EGT information available from your EFIS.
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  #14  
Old 02-21-2016, 10:26 PM
aerhed aerhed is offline
 
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Yah, they're not airtight. If its worn bad you will see it most at like 3/4 throttle. Not talking slight movement, but actual rattle the shaft with your fingertips worn. The bubbles around the screw are from your air supply blowing back through the idle ports (little tiny holes by the throttle plate edge at idle).
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  #15  
Old 02-22-2016, 06:59 AM
Jbon Jbon is offline
 
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Thanks guys, I'll do some more research on mixture. Good to know that leakage there is normal. The engine seems to run okay. No stumble on acceleration, or fouled plugs.

What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag. The guy at Electroair indicated that because of the higher efficiency of the EIS, I shouldn't see much of any drop. When I mentioned my mixture issue, he said to get that squared away first. Again, plugs look good. The leads look pretty good too, but they are nearly 20 years old. Any thoughts there?
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  #16  
Old 02-22-2016, 09:33 AM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Jbon View Post
What started this whole thing was trying to figure out why I'm getting a 150 RPM drop on the Electroair ignition during run up, vs 50 on the mag.
Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.

Larry
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  #17  
Old 02-22-2016, 04:35 PM
Jbon Jbon is offline
 
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Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.
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  #18  
Old 02-22-2016, 08:55 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by Jbon View Post
Makes sense. The bottom plugs (EIS) look good, as do the leads, though they are 19 years old and have spent a lot of that time sitting. Perhaps swapping the upper/lower plugs would be a good place to start, followed by replacing the leads.
Unlike choosing a wife, looks mean nothing here. By test, I mean with an Ohmmeter.

Larry
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  #19  
Old 02-22-2016, 08:59 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by lr172 View Post
Idle mixture would be pretty far down the list of likely things to cause that. If taking the mag off-line, I would expect a small to minimal drop in RPM. THe first place that I would be looking at is timing (Mag and EI) and the second would be testing each plug and wire on both sides.

Larry
Just thought about this more. Are these mag drops done at 1600-1800 RPM? Idle mixture has zero impact after about 1200 RPM. I would consider not taking too much advice from the EI guy who suggested you sort out your idle mixture for behavior at 1800 RPM.

Larry
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  #20  
Old 02-23-2016, 10:50 AM
Jbon Jbon is offline
 
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Agreed. That makes sense. Next step is to break out the ohm meter. These leads are "Autolite". Carbon, I assume. What resistance per foot should I be looking for?

Thanks
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